1. Field of the Invention
This invention relates to the field of flame-resistant polyimide foam insulation prepared from aromatic polyisocyanates and aromatic polycarboxylic compounds.
2. Description of the Prior Art
The preparation of polyimide foams from polyisocyanates and aromatic polycarboxylic acid derivatives has generally been accomplished in the presence of various catalyst and with suitable heating. While the foams thus obtained possess excellent fire-retarding properties, their application is restricted to situations where the required heating is practical.
More recently, a process has been developed to produce a flame-resistant, thermally stable polyimide foam without requiring external heating. As described in U.S. Pat. No. 4,184,021, an aromatic polyisocyanate and an aromatic polycarboxylic compound are reacted in the presence of furfuryl alcohol and a strong inorganic acid. The alcohol and the acid produce a vigorous exothermic reaction which provides all the heat necessary for the formation of the polyimide foam.
The present invention is an improvement over the process described in the above mentioned patent. A recognized problem with polyimide foams is the need to reduce their friability while still maintaining their high flame resistant quality. This problem is recognized in U.S. Pat. No. 3,772,216 which discloses the addition of reinforcing fibers such as silicon dioxide fibers 1/32 to 1/4 inch long to reduce the polyimide foam's friability.